Solution to the problem of the poor cyclic fatigue resistance of bulk metallic glasses

Abstract

The recent development of metallic glass-matrix composites represents a particular milestone in engineering materials for structural applications owing to their remarkable combination of strength and toughness. However, metallic glasses are highly susceptible to cyclic fatigue damage, and previous attempts to solve this problem have been largely disappointing. Here, we propose and demonstrate a microstructural design strategy to overcome this limitation by matching the microstructural length scales (of the second phase) to mechanical crack-length scales. Specifically, semisolid processing is used to optimize the volume fraction, morphology, and size of second-phase dendrites to confine any initial deformation (shear banding) to the glassy regions separating dendrite arms having length scales of Ϸ2 m, i.e., to less than the critical crack size for failure. Confinement of the damage to such interdendritic regions results in enhancement of fatigue lifetimes and increases the fatigue limit by an order of magnitude, making these ''designed'' composites as resistant to fatigue damage as high-strength steels and aluminum alloys. These design strategies can be universally applied to any other metallic glass systems.composites ͉ damage confinement ͉ endurance limit ͉ semisolid processing M onolithic bulk metallic glasses (BMGs) have emerged over the past 15 years as a class of materials with unique and unusual properties that make them potential candidates for many structural applications (1). These properties include their near theoretical strengths combined with high formability, low damping, large elastic strain limits, and the ability to be thermoplastically formed into precision net shape parts in complex geometries (2, 3), all of which are generally distinct from, or superior to, corresponding crystalline metals and alloys. However, monolithic BMGs can also display less desirable properties that have severely restricted their structural use. In particular, properties limited by the extension of cracks, such as ductility, toughness, and fatigue, can be compromised in BMGs by inhomogeneous plastic deformation at ambient temperatures where plastic flow is confined in highly localized shear bands (4, 5). Such severe strain localization with the propagation of the shear bands is especially problematic under tensile stress states where catastrophic failure can ensue along a single shear plane with essentially zero macroscopic ductility (6, 7). Consequently, resulting plane-strain K Ic fracture toughnesses in monolithic BMGs are often low (Ϸ15-20 MPa ͌ m), as compared with most crystalline metallic materials, although they are an order of magnitude larger than those for (ceramic) oxide glasses (8, 9). If such strain localization is suppressed such that plastic flow is allowed to be extensive, for example, by blunting the crack tip, then damage would be distributed over larger dimensions with toughness values increasing to Ϸ50 MPa ͌ m or more (8, 10). Whereas some metallic glasses appear to be intrinsically brittle in thei...

Near-threshold fatigue crack growth in bulk metallic glass composites

“…10. This is a similar result to what has been shown in quasistatic fracture toughness tested samples 18 and fatigue samples 20 of DH1. Since the specimen size is small, both in terms of thickness as well uncracked ligament length, a plastic hinge develops and the data obtained are no longer valid in the context of fracture mechanics.…”

“…A number of factors such as the angle of crack incidence, the elastic mismatch across the interface, and the ratio of interfacial toughness determine the path of the crack. 20 It is inferred from scanning electron micrographs that at higher incidence angles the crack penetrates through the dendrites, while at lower incidence angles the crack gets deflected and passes through interface of dendrites and matrix. Fractographs obtained from the samples that fractured into two pieces are shown in Fig.…”

“…Recently, Launey et al 20 examined the unnotched fatigue response (stress-life or S-N approach) of a semisolidly processed BMG matrix composite with nominal composition of Zr 40 Ti 34 Nb 7.6 Cu 6.4 Be 12.5 , developed by Hofmann et al, 15 and show that it has remarkably higher fatigue limit than monolithic BMGs, and even better than some steel and aluminum alloys. However, reliable structural design strategies require characterization of a material through the fracture mechanics approach (also referred to as defect tolerant approach), as the S-N approach is highly sensitive to experimental parameters such as the type of loading applied, surface conditions of the specimens, and the casting defects in them.…”

Corrosion Fatigue Studies on a Bulk Glassy Zr-Based Alloy under Three-Point Bending

“…While Vitreloy 105 shows probably the highest strength-normalized fatigue endurance of ~0.24 (Naleway et al, 2013), which is also comparable to some high-strength steels, many attempts have been taken to improve the fatigue performance of BMGs. For example, Launey et al (2009) developed glass matrix composites with microstructural length scales of the second phase according with mechanical crack length scales (Launey et al, 2009). A retarded growth of small flaws acting as preferential crack initiation sites was achieved, and subsequent failure by single shear band opening could be impeded.…”

Fracture toughness and crack-resistance curve behavior in metallic glass-matrix composites

“…By introducing a second phase in form of crystalline dendrites and by matching microstructural length scales ͑i.e., interdendritic spacing͒ to the mechanical length scales ͑i.e., critical crack size for failure͒, nonlocalized plasticity in metallic glasses can be enhanced significantly. [5][6][7] Indeed, the recent development of in situ bulk metallic glass-matrix composites has shown that provided this phase acts to arrest shear-band propagation over appropriate size scales, [8][9][10] the problems of poor ductility, toughness, and fatigue resistance can be mitigated.…”